I now have nVidia GeForce 6600LE 256M
I want to get better card, because i'm unable (sometimes) to watch HD movies on my screen (1276x1024, i'm also considering upgrading monitor to 19'-21')

The reason why i'm considering upgrading to ATI is because AMD have released documentation/specification for ATI video cards, and open source community is writing drivers for it....

The question is:
* How far ready is ATI open source driver? Is it usable?
* Which ATI card would be best for my purpose (mostly watching HD videos). I don't play games (well, maybe one day, when i get real pissed i will decide to play Nexuiz), so 3D graphics ain't important (however it would be good to have 3D support just in case )

I would also like to hear what does ATI users have to say on this matter (how driver works, what works, what not, how fast.... etc...)

First, in the FreeBSD kernel and the ports tree, you will currently find support for all radeons up to, and including the x850. These are the r100 through r400 family cards.

r500 (up to the x1950) support has been added to Mesa, and will be available in the ports tree after FreeBSD 7.1 is released. While this doesn't pertain to you, Xpress chipset support (for some but not all Xpress GPUs) is also working in Mesa 7.2 and will be available via ports after 7.1. It does work now if you compile Mesa yourself, though then you need a newer X server for AIGLX support. The DRM kernel code will not be available in FreeBSD 7.1, but installing it from git is pretty trivial.

2D and 3D acceleration for r600 and newer cards is *not* available in any open source drivers yet. The specifications have not yet been released without an NDA. Development is on-going, but behind closed doors, and AMD is attempting to clean up the specifications as quickly as possible to make them available. The radeonhd/radeon drivers provide modesetting for these cards.

The r500 3D support is good enough to play many games, and use compiz (or the kde4 effects). I can play ut2004 and even doom3 (though it requires using the ARB render path), after I copied the drivers over from a linux installation to my /compat/linux tree. I can watch 720p HD video trailers, but have not tried anything higher.